I. Field of the Invention
This disclosure relates generally to apparatus and methods for position estimation of a wireless device, and more particularly to determining a time of arrival in RTT (round-trip time) systems or OWT (one way time) if the systems are synchronous.
II. Background
To assist in position estimation, a mobile device may capture received signal strength indication (RSSI) measurements from three or more access points. A server or the mobile device itself may apply trilateration to these RSSI measurements using a distance model based on RSSI to estimate a position of the mobile device. Unfortunately, trilateration with RSSI measurements results in a high level of uncertainty in position estimation because of the uncertainty of the RSSI measurement themselves and dependency on an accurate distance model based on RSSI measurements.
Round-trip time (RTT) or one way time (OWT) measurements advantageously have a much lower level of distance uncertainty than the RSSI measurements. RTT measurements record a round-trip time starting with an initial signal being transmitted, accounting for remote transceiver processing delays, and ending with a final signal being received. For example, a signal is transmitted from a mobile device to an access point and back to the mobile device. Though several uncertainties exist (such as processing delays with the remote transceiver) with RTT measurement, these variables may be determined or estimated. A server or a mobile device may use the RTT measurements in trilateration to more accurately estimate the position of the mobile device.
Unfortunately, determining when a signal is received may be inaccurate up to 20% or 25% of a width of an OFDM symbol, therefore, other means are used to determine RTT. A more precise method is needed to eliminate this inherent and unknown offset so this windowing method may be used.